[AArch64] Use scvtf fbits option where appropriate

[AArch64] Use scvtf fbits option where appropriate

gcc/ChangeLog:
2019-08-19  Joel Hutton  <Joel.Hutton@arm.com>

	* config/aarch64/aarch64-protos.h (aarch64_fpconst_pow2_recip): New prototype
	* config/aarch64/aarch64.c (aarch64_fpconst_pow2_recip): New function
	* config/aarch64/aarch64.md (*aarch64_<su_optab>cvtf<fcvt_target><GPF:mode>2_mult): New pattern
	(*aarch64_<su_optab>cvtf<fcvt_iesize><GPF:mode>2_mult): New pattern
	* config/aarch64/constraints.md (Dt): New constraint
	* config/aarch64/predicates.md (aarch64_fpconst_pow2_recip): New predicate

gcc/testsuite/ChangeLog:
2019-08-19  Joel Hutton  <Joel.Hutton@arm.com>

	* gcc.target/aarch64/fmul_scvtf_1.c: New test.

From-SVN: r274676